1. Field of the Invention
The present invention generally relates to a downhole production operation. More particularly, the invention relates to seismic monitoring. More particularly yet, the invention relates to an adjustable deployment apparatus for an in-well seismic station.
2. Description of the Related Art
In situ measurement of the seismic wavefield generated by a surface source is becoming an increasingly common geophysical surveying procedure. This measurement, known as vertical seismic profiling (VSP), furnishes the opportunity to investigate wave propagation in the shallow earth and to estimate the seismic values describing the lithologic section. VSP can be used within a single well or can be used in multiple wells such as a cross-well arrangement. Generally, VSP includes an array utilizing of seismic stations. Each seismic station includes a plurality of sensors, such as acoustic and/or pressure sensors, which are arranged within the wellbore to measure the acoustic waves between the wellbore and the surface. The sensors provide seismic data that is analyzed by a seismic data processor to determine the subsurface stratigraphy in relation to the seismic reflections measured at the surface.
It is generally preferred to permanently position the sensors within the well without substantially interfering with normal production operation of the well. Various deployment techniques exist to mechanically couple sensors to a borehole structure, such as the production tube, the well casing, or a production packer. For instance, one deployment technique involves arranging the sensors outside the casing prior to a cementation operation. Thereafter, the sensors are surrounded by cement as the cement is injected into an annular space formed between the casing and the borehole of the well. Embedding the sensors in this manner is beneficial in that acoustic waves used in the seismic analysis can easily travel to the sensors without attenuation. In addition, different types of acoustic waves (e.g., shear waves) can be sensed using this method. However, mechanically coupling the sensors to the outside of the casing can, in some cases be difficult to perform, as in the case of complex wellhead configurations, expandable casing, multiple casing strings, tight casing annular constraints, or may limit the placement of sensors in the borehole. For example, the casing may not extend deep enough to position the sensors in the desired location.
In another deployment technique employed in vertical seismology, the sensors are only temporarily located within the well. In this deployment technique, the sensors are used to take readings and then retrieved from the well. In addition, the position of the sensors can be changed within the well to take into account alterations of the earth strata under analysis, resulting from production of effluents. However, deployment or retrieval of temporary sensors disrupts production (or injection) from the well, which can be particularly costly if measurements are periodically made to assess strata conditions over a given time period. Furthermore, preparing the sensors for insertion into the well, properly positioning the sensors, and retrieving the sensors requires tedious preparation and execution and can pose additional risk to the wellbore or wellbore completion if equipment is inadvertently lodged or dropped into the wellbore.
Recently, another deployment technique has been developed to actively couple an in-well seismic sensor to the casing of the well by using a clamp mechanism as disclosed in co-owned U.S. Provisional Patent Application having a Ser. No. 60/416,932, filed on Oct. 6, 2002, which is entitled CLAMP MECHANISM FOR IN-WELL SEISMIC SENSOR and is herein incorporated by reference. Generally, a plurality of clamp mechanisms carrying an array of sensors are deployed into the well attached to a string of production tubing. More specifically, an external clamp is employed to externally clamp the sensor to the string of tubing. Each clamp mechanism is attached to the string of production tubing at a respective joint between two production tubulars and subsequently lowered into the wellbore along with the string of production tubing. After the clamp mechanisms are positioned at their respective desired locations, a release mechanism in each clamp mechanism is activated thereby displacing the sensor radially outward until the sensor is coupled to the casing. Once coupled to the casing, the sensor is substantially acoustically decoupled from the clamp mechanism and production tubing. There are problems associated with actively coupling an in-well seismic sensor using the clamp mechanism. For example, the external clamp can be dislodged during deployment of the string of production tubing resulting in damage to the sensor system or in the tubing becoming lodged in the wellbore. In another example, a problem may arise due to length variations of the production tubulars. For instance, each production tubular has a length tolerance of several inches. This means that the spacing between each sensor in the array may vary several inches, thereby making it difficult to properly position the sensors in the wellbore. Furthermore, the rotational alignment between each clamp mechanism may be compromised due to the threaded connection between each production tube.
There is a need therefore for a seismic station deployment apparatus that is axially adjustable to compensate for length variations of the production tubing while remaining fixed to the production tubing. There is a further need therefore for a seismic station deployment apparatus that is rotationally adjustable. There is yet a further need for an effective deployment technique for placing a seismic array in a well. Further, there is a need for a cost effective means of deploying a seismic station on a casing string.